1. Field of the Invention
This invention relates to an improvement in a method of and a device for resin sealing semiconductor devices.
2. Description of the Prior Art
FIG. 1 shows a conventional device for resin sealing semiconductor chips attached to a lead frame. This device is disclosed in Japanese Laid-Open Patent Application No. 26827/87. As shown in the drawing, a chase block 1 is equipped with a plurality of cavities (not shown) accommodating semiconductor devices (not shown) mounted on lead frames. This chase block 1 is of the same height as a chamber block (not shown) equipped with a well-known potting section (not shown) for injecting plastic as well as a runner section (not shown). The chamber block and chase block 1 are fastened to a retention board 2 by means of bolts. Heaters 3 for heating the chamber block (not shown) and chase block 1 and keeping them warm are inserted into retention board 2. Spacer blocks 4 supporting the retention board 2 on the base 7 are provided with guide grooves 4a for fixtures 8 for fixing the entire device mentioned above to a well-known press slide frame 9. A post 6 supports the retention board 2 on the base 7. Posts 5 on both sides of the post 6 also support the retention board 2. The post 6 is higher than the posts 5, which are higher than the spacer blocks 4. These differences in height are shown as .delta..sub.1, .delta..sub.2 in FIG. 2. In regard to the retention board 2 which serves as a continous beam and the spacer blocks and the posts 5, 6 as fulcrums, the differences .delta..sub.1, .delta..sub.2 may be determined by calculating the height of the posts 5 and 6 from the reaction force and spring constant at each fulcrum (i.e. at the spacer blocks 4 and the posts 5,6) under the press clamping force. The condition for this calculation is that the height of the spacer blocks 4 and the posts 5, 6 under the press clamping force be the same. The base 7, which supports retention board 2 through the spacer blocks 4 and posts 5, 6, contains an insulating plate 7a for preventing the heat of the heaters 3 from being transmitted to the press slide frame 9. The chase block 1, chamber block, retention board 2, spacer blocks 4, posts 5, 6 and base 7 form a lower die. An upper die 10 of the resin sealing device has substantially the same construction as the lower die described above. Reference numeral 11 denotes a press platen and 12 a parting surface of said chase block 1.
In a device thus constructed, the press clamping force during a resin sealing operation causes a uniform load to be applied to the parting surface 12. This load is received by the retention board 2, and is then distributed to the posts 5 and 6. The load thus distributed is then received by the base 7 and finally received by the press slide frame 9. Since the height of the posts 5 and 6 are so determined that the upper ends of the spacer blocks 4 and the posts 5, 6 are in one plane after the deflection caused by the press clamping force, the parting surface 12 of the chase block 1 can be, as shown in FIG. 3, kept planar. The posts 5, 6 and the spacer blocks 4 are formed beforehand with dimensions based on the calculation formula. Then, after assembling the device, the height of the posts 5, 6 and the spacer blocks 4 are adjusted by gauging while checking how the die faces meet each other. As stated above, the heaters 3 inserted into the retention board 2 heat the chase block 1 and keep it warm. Transmission and radiation of the heat generated in the process to the base 7 through the retention board 2, the posts 5, 6 and the spacer blocks 4 is prevented by the insulating plate 7a provided in the base 7. In other words, the insulating plate 7a prevents the heat transmission to the base 7 and thence to the press slide frame 9 that would otherwise take place through the posts 5, 6 and the spacer blocks 4 which are attached to both the retention board 2 and the base 7. The experimental temperature is 180.degree. C. on the parting surface 12 and 120.degree. C. on the surface of the base 7 on the side of the retention board 2. The temperature on the opposite surface of the base 7, on the other hand, is 50.degree. C. because of the presence of the insulating plate 7a. In this way, the insulating plate 7a prevents the heat on the parting surface 12 from escaping.
As described above, conventional devices for resin sealing semiconductor devices absolutely need a heat insulating plate because the heat in the retention board 2 tends to be transmitted to the base through the posts 5, 6 and the spacer blocks 4. As a result, such conventional devices are rather expensive. Further, the uneven deformation of the insulating plate that occurs over time may give rise to a clearance between the parting surfaces of the two opposing chase blocks; consequently, lead frames become subject to the generation of resin burrs.
Moreover, the height of the posts and the spacer blocks must be adjusted by gauging while checking how the die parting surfaces meet each other during operation, resulting in a complicated assembly process.